A patient requires IOL power calculation before cataract surgery. Axial length is 26.5 mm (high myopia), and keratometry is 44.0D. Using a 3rd generation formula (e.g., SRK/T), a hyperopic surprise is more likely than with 4th generation formulas. The reason is:
- A SRK/T overestimates the effective lens position (ELP) for longer eyes, thereby underpowering the IOL prescription and causing a hyperopic shift post-operatively ✓
- B SRK/T underestimates the effective lens position in long eyes, leading to a predicted thicker IOL than required
- C SRK/T overestimates K readings causing extra minus to be prescribed
- D All formulas perform equally in axial lengths > 26 mm
Correct answer: A. SRK/T overestimates the effective lens position (ELP) for longer eyes, thereby underpowering the IOL prescription and causing a hyperopic shift post-operatively
Explanation
In high axial myopia (AL > 26 mm), 3rd generation formulas like SRK/T tend to overestimate the ELP (assumed deeper IOL position) relative to the actual post-operative ELP. This leads to selection of an IOL power that is too low (more negative or less positive), resulting in hyperopic surprise. Fourth-generation formulas (Barrett Universal II, Haigis, Olsen) and newer AI-based formulas (Kane, EVO, PEARL-DGS) better account for ELP in extreme axial lengths and have significantly lower prediction errors in high myopes.
Reference: Khurana Comprehensive Ophthalmology, 7th ed.
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Written and medically reviewed by the StethoPrep medical team.